Fiber optic transducers, fiber optic accelerometers and fiber optic sensing systems

ABSTRACT

A fiber optic transducer is provided. The fiber optic transducer includes a fixed portion configured to be secured to a body of interest, a moveable portion having a range of motion with respect to the fixed portion, a spring positioned between the fixed portion and the moveable portion, and a length of fiber wound between the fixed portion and the moveable portion. The length of fiber spans the spring. The fiber optic transducer also includes a mass engaged with the moveable portion. In one disclosed aspect of the transducer, the mass envelopes the moveable portion.

RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.14/789,537, filed Jul. 1, 2015, which is a continuation of U.S.application Ser. No. 13/263,342, filed on Jun. 25, 2012, now U.S. Pat.No. 9,097,505, which application claims the benefit of priority toPCT/US2010/053659 filed on Oct. 22, 2010, which application claims thebenefit of priority to U.S. Provisional Patent Application Ser. No.61/279,607, filed on Oct. 23, 2009, the contents of which areincorporated in this application by reference.

TECHNICAL FIELD

This invention relates generally to the field of fiber optic sensingsystems, and more particularly, to improved fiber optic transducers,accelerometers, interferometers, and improved fiber optic sensingsystems.

BACKGROUND OF THE INVENTION

Fiber optic sensing systems are widely used for sensing disturbances(e.g., motion, acceleration, sound, etc.). Such fiber optic sensingsystems often include a transducer for converting the disturbance into aphase change of light in an optical fiber.

Such transducers suffer from a number of deficiencies. For example,certain fiber optic sensing applications have spatial restrictions whichlimit the applicability of certain transducer designs. Further, theenvironment in which the transducers (and other optical elements of thefiber optic sensing systems) are used may require sensitivity andcontrol not obtained or available from many conventional transducers.Further still, the operation of many optical transducers is adverselyaffected by disturbances along differing axes of motion.

Thus, it would be desirable to provide improved optical transducers,fiber optic accelerometers, and related fiber optic sensing systems toaddress these and other issues.

BRIEF SUMMARY OF THE INVENTION

According to an exemplary embodiment of the present invention, atransducer is provided. The transducer includes a fixed portionconfigured to be secured to a body of interest, a moveable portionhaving a range of motion with respect to the fixed portion, a springpositioned between the fixed portion and the moveable portion, and alength of fiber wound between the fixed portion and the moveableportion. The length of fiber spans the spring. The transducer alsoincludes a mass engaged with the moveable portion. The transducer may beincluded as part of an accelerometer. The transducer/accelerometer mayinclude various additional features that are not mutually exclusive withrespect to one another. For example, the mass may be configured toenvelope the moveable portion (as well as other portions of thetransducer). Further, the mass may house certain optical elements (e.g.,a reflector, a fiber optic coupler, etc.) of the accelerometer. Furtherstill, certain of the elements of the transducer (e.g., the fixedportion, the moveable portion, and the spring) may be formed from aunitary piece of material.

The inventive transducers (and inventive accelerometers) may beincorporated into fiber optic sensing systems having additional opticalelements. Exemplary fiber optic sensing systems include Sagnacinterferometer sensing systems, Michelsen interferometer sensingsystems, and Fabry Perot interferometer sensing systems.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, but are notrestrictive, of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best understood from the following detailed descriptionwhen read in connection with the accompanying drawings. It is emphasizedthat, according to common practice, the various features of the drawingsare not to scale. On the contrary, the dimensions of the variousfeatures are arbitrarily expanded or reduced for clarity purposes.Included in the drawings are the following figures:

FIG. 1 is a block diagram illustration of a fiber optic accelerometer inaccordance with an exemplary embodiment of the present invention;

FIGS. 2A-2H are cross-sectional block diagram views of transducers inaccordance with various exemplary embodiments of the present invention;

FIGS. 2I-2J are top and bottom perspective views of a transducer inaccordance with an exemplary embodiment of the present invention;

FIG. 3A is a block diagram view of a linearized Sagnac interferometer inaccordance with an exemplary embodiment of the present invention;

FIG. 3B-3C are block diagram views of Michelsen interferometers inaccordance with various exemplary embodiments of the present invention;

FIG. 4A is a block diagram view of a plurality of multiplexed Sagnacinterferometers in accordance with an exemplary embodiment of thepresent invention;

FIG. 4B is a block diagram view of a plurality of multiplexed FabryPerot interferometers in accordance with an exemplary embodiment of thepresent invention;

FIG. 5 is an exploded view of a transducer in accordance with anexemplary embodiment of the present invention;

FIGS. 6A-6B are top and perspective views of a hinge of the transducerof FIG. 5;

FIG. 6C illustrates exemplary mounting of the hinge of FIGS. 6A-6B;

FIG. 7A is a sectional block diagram view of a transducer in accordancewith an exemplary embodiment of the present invention; and

FIGS. 7B-7C are block diagram views of damping modes of the transducerof FIG. 7A.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates generally to transducers, accelerometers (i.e.,interferometers), and fiber optic sensing systems for sensing physicaldisturbances (e.g., motion, acceleration, perturbations, etc.) of a bodyof interest. As will be appreciated by those skilled in the art, a fiberoptic accelerometer (sometimes referred to as a fiber optic sensor or afiber optic interferometer) is an element of a system for measuringphysical motion of a body of interest using fiber optic technology. Theaccelerometer includes a transducer that converts a physical disturbanceof the body of interest into a change in strain applied to a length ofoptical fiber of the transducer.

Referring to FIG. 1, fiber optic accelerometer 10 includes opticalsource 20 (e.g., an LED, an SLED, a laser, etc.), sensor 30 (e.g., aninterferometer), optical receiver 60 (e.g., an optical detector such asa photodetector), optical fiber 40 for transmitting light from opticalsource 20 to sensor 30, and optical fiber 50 for returning light fromsensor 30 to receiver 60. Sensor 30 includes a transducer which convertsmechanical or physical motion (such as acceleration) to a change in thestrain (e.g., longitudinal strain) in an optical fiber. Sensor 30 alsoincludes other optical elements for converting the change in strain to achange in the phase of light that passes through the optical fiber 40,50. FIGS. 2A-2H illustrate various transducers which may be included insensor 30 of FIG. 1.

FIG. 2A illustrates transducer 200 including fixed portion/mandrel 205and moveable portion/mandrel 210 separated by spring 230. Optical fiber150 (illustrated as a dashed box between mandrels 205, 210) is woundaround fixed mandrel 200 and moveable mandrel 210, where spring 230applies a biasing tension to optical fiber 150 (e.g., with an exampletension of the wound length of optical fiber 150 being approximatelybetween 0.1-4.0 newtons). Optical fiber 150 may be fixed to mandrels205, 210 at the ends of the wound portion using an adhesive (not shown)if desired (e.g., epoxy, acrylate adhesive, etc.). Mass 220 is securedto moveable mandrel 210 (e.g., using fasteners such as screws, using arigid adhesive, etc.). Alternatively, mass 220 and moveable mandrel 210may be formed from a unitary piece of material. Example materials ofmandrels 205, 210 (and mass 220) are metals (e.g., aluminum, stainlesssteel, brass, etc.) and plastics (e.g., polycarbonate). Hinges 240 a,240 b (which are provided between fixed mandrel 205 and mass 220) limitthe range of motion of moveable mandrel 210 (and mass 220) to adirection substantially along axis labeled as axis “Y”, where axis “Y”is a single linear degree of freedom that is substantially parallel tointerior wall portion 220 w of enveloping mass 220 (and is substantiallyparallel to an imaginary line connecting mandrels 205 and 210). Theexemplary hinges 240 a, 240 b shown in the cross-sectional view of FIG.2A are circular hinges similar in function to hinge 210 n describedbelow with respect to FIG. 5.

Fixed mandrel 205 is rigidly attached to body of interest 202, or may berigidly attached to body of interest 202 through a base plate or otherstructure (not shown). When body of interest 202 undergoes acceleration(or other physical disturbance) in space, the result is relative motionbetween fixed mandrel 205 and moveable mandrel 210/mass 220. Thisrelative motion changes the longitudinal strain within optical fiber150. As provided above, this change in the longitudinal strain inoptical fiber 150 is converted to a change in the phase of light as thelight passes through optical fiber 150.

In transducer 200 shown FIG. 2A (and in various other exemplarytransducers such as those shown in FIGS. 2B-2E and FIGS. 2G-2H), mass220 envelopes at least one of fixed mandrel 205, spring 230, and/or thewound length of fiber 150 within at least one position (or within everyposition) within the range of motion of moveable portion 210. That is,mass 220 has a substantially cylindrical shape where inner side walls220 w of the cylindrical shape surround (i.e., envelope) at least one offixed mandrel 205, spring 230, and/or the wound length of fiber 150. Byenveloping such elements with mass 220, various benefits are provided.For example, the mass provided per volume of the transducer isrelatively high because of the enveloping shape of mass 220. Further,the enveloping shape assists in controlling the center of gravity of themass 220. Further still, the enveloping shape of the mass 220 reducessensitivity to off-axis excitation. The features and details describedabove with respect to FIG. 2A are applicable to the exemplaryembodiments shown in FIGS. 2B-2H unless indicated otherwise.

FIG. 2B illustrates transducer 200 a including fixed mandrel 205 a,moveable mandrel 210 a, and spring 230 a separating mandrels 205 a, 210a. Optical fiber 150 a is wound around fixed mandrel 205 a and themoveable mandrel 210 a. Mass 220 a is secured to moveable mandrel 210 a,or alternatively, mass 220 a, and moveable mandrel 210 a may be formedfrom a unitary piece of material. Fixed mandrel 205 a is rigidlyattached to body of interest 202 a, or may be rigidly attached to bodyof interest 202 a through a base plate or the like. When body ofinterest 202 a undergoes acceleration (or another physical disturbance)in space, the result is relative motion between fixed mandrel 205 a andmoveable mandrel 210 a/mass 220 a. This relative motion changes thelongitudinal strain within optical fiber 150 a, where such longitudinalstrain in optical fiber 150 a is converted to a change in the phase oflight as the light passes through optical fiber 150 a. FIG. 2B differsfrom FIG. 2A primarily in terms of the shape of certain of the elements;however, the function of the elements is substantially the same.

FIG. 2C is substantially similar to FIG. 2B, except that the elementsillustrated have reference numerals ending with “b” instead of “a”. Theprimary difference between FIG. 2C and FIG. 2B is that in FIG. 2C spring230 b is a bent sheet metal spring (as opposed to the coil compressionstyle spring shown in FIG. 2B) that can also provide a hinge function,similar to hinges 240 a, 240 b described above with respect to FIG. 2A,as well as a biasing spring function. Of course, other types of springmembers are contemplated.

As will be appreciated by those skilled in the art, the mass does notneed to be a cylindrical enveloping mass as shown in FIGS. 2A-2C, thatis, other shapes are contemplated. FIGS. 2D-2E are substantially similarto FIG. 2B, except that the elements illustrated have reference numeralsending with “c”/“d” instead of “a”; however, the shapes of theenveloping mass 220 c and 220 d is varied in FIGS. 2D-2E. In FIG. 2D,enveloping mass 220 c has a biconical shape (as opposed to a cylindricalshape as in FIG. 2B). In FIG. 2E, enveloping mass 220 d has a sphericalshape. As with mass 220 (described above in connection with FIG. 2A),mass 220 c/220 d envelopes other portions of the transducer within therange of motion (or at least one position within the range of motion) ofthe moveable portion of the transducer, as desired. Other exemplaryshapes of the enveloping mass include a conical shape, a rhombic shape,amongst others.

As will be appreciated by those skilled in the art, each of thetransducers illustrated in FIGS. 2A-2H may be included in an opticalinterferometer (also referred to as a “sensor”). Such an opticalinterferometer includes optical elements that perform functionsincluding the conversion of the change in the phase of light passingthrough the fiber in the transducer to a change in optical intensity.According to certain exemplary embodiments of the present invention,certain of these optical elements may be provided within the masssecured to (or integrated with) the moveable mandrel. FIG. 2F issubstantially similar to FIG. 2B, except that the elements illustratedhave reference numerals ending with “e” instead of “a”, and that themass 220 e has a different shape (and an additional function) ascompared to mass 220 a. Mass 220 e extends above moveable mandrel 210 eat a terminal end of the transducer (and in certain embodiments, aterminal end of the accelerometer). Mass 220 e defines a volumeconfigured to house at least one optical element of the accelerometer.Exemplary optical elements that may be housed within mass 220 e includea fiber optic coupler, a reflector, an optical source (e.g., a lightsource), an optical receiver/detector, an optical depolarizer, a delaycoil of optical cable, and a phase modulator. Any portion of theseelements, and/or additional elements, may be housed within mass 220 e asis desired in the given application. Of course, the shape of mass 220 eillustrated in FIG. 2F is exemplary in nature—other shapes arecontemplated. Further, while mass 220 e is not illustrated as an“enveloping” mass as described above, the present applicationcontemplates a mass combining the features of housing optical elementsas well as including the enveloping feature described above.

FIG. 2G illustrates transducer 200 f where each of fixed mandrel 205 f,spring 230 f, and moveable mandrel 210 f are formed from a unitary pieceof material. Enveloping mass 220 f may be secured to moveable mandrel210 f, or mass 220 f may be included in the unitary piece of materialwith moveable mandrel 210 f. As with the previously describedembodiments, fixed mandrel 205 f is rigidly attached to body of interest202 f, or may be rigidly attached to body of interest 202 f through abase plate or the like. A length of optical fiber 150 f is wound aroundfixed mandrel 205 f and the moveable mandrel 210 f. Otherwise, thefunction of transducer 200 f is substantially similar to that describedabove with respect to FIG. 2A.

FIG. 2H is substantially similar to FIG. 2B, except that the elementsillustrated have reference numerals ending with “g” instead of “a”. Theprimary difference between FIG. 2H and FIG. 2B is that in FIG. 2H thespring function is provided by compressive bellows element 230 g.Element 230 g may have a hollow circular cross section, and may bedefined by one or more metallic sub-elements, arranged to provide aspring function, but also creating lateral (and/or) torsional stiffness,as is well known by those skilled in the art.

FIGS. 2I-2J are top and bottom perspective views of a transducer such asthat shown in FIGS. 2A-2H, excluding obvious distinctions such as theshape of the enveloping mass 220. Each of FIGS. 2A-2H includes a lengthof optical fiber (e.g., length of fiber 150, 150 a, 150 b, etc.) woundbetween the fixed mandrel and the moveable mandrel; however, none ofFIGS. 2A-2H illustrates the fiber entering (or exiting) the transducer.That is, in FIGS. 2A-2H only that portion of the fiber wound between thefixed mandrel and the moveable mandrel is shown. FIGS. 2I-2J illustratefiber 150 n entering (and exiting) transducer 200 n. Transducer 200 nincludes top plate 204 n secured to enveloping mass 220 n and themoveable mandrel. Top plate 204 n is also secured to a moveable mandrel(not visible in FIGS. 2I-2J). Transducer 200 n also includes mountingplate 206 n, bottom plate 208 n (e.g., a retaining ring), and circularhinge 210 n. Bottom plate 208 n is used to secure circular hinge 210 nto mass 220 n. Mounting plate 206 n secures the inner region of circularhinge 210 n to a fixed mandrel (not visible in FIGS. 2I-2J). Mountingplate 206 n may also be used to secure transducer 200 n to a body ofinterest (or to an interposing structure) through mounting holes 212 n.Additional features of transducer 200 n will be described below inconnection with the exploded view provided in FIG. 5.

As provided above, a transducer may be included as part of aninterferometer, where the interferometer converts a change in theoptical phase of light propagating along the optical fiber 150 withinthe transducer to a change in the optical intensity of the light leavingthe interferometer. Transducers according to the present invention maybe utilized in connection with any of a number of types ofsensors/interferometers, and may be used in any of a number of variedapplications. Exemplary sensors/applications for the transducers includefiber optic sensing systems. Exemplary fiber optic sensing systemsinclude Sagnac interferometer sensing systems, Michelsen interferometersensing systems, Fabry Perot interferometer sensing systems, andMach-Zender interferometer sensing systems. FIG. 3A illustrates alinearized Sagnac sensing system including a single sensor, where such aSagnac interferometer may be desirable because of a relatively smallsize and low cost. FIGS. 3B-3C illustrate Michelsen sensing systems,each including a single sensor. FIG. 4A illustrates a multiplexed Sagnacsensing system including a plurality of sensors. FIG. 4B illustrates amultiplexed Fabry Perot sensing system including a plurality of sensors.

Referring specifically to FIG. 3A, a fiber optic sensing system includesinterferometer 300 (i.e., sensor 300, which is a linearized Sagnacinterferometer) as well as optical source 302 and optical receiver 304.Interferometer 300 includes optical coupler 310 (e.g., a 3×3 opticalcoupler) for receiving an optical signal (e.g., light) from opticalsource 302, and for transmitting the optical signal out ofinterferometer 300 to optical receiver 304. First output lead 310 a ofoptical coupler 310 is connected to input lead 320 a of delay coil 320.Output lead 320 b of delay coil 320 is connected to first input lead 330a of optical coupler 330 (e.g., a 1×2 fiber optic coupler 330). Secondoutput lead 310 b of optical coupler 310 is connected to input lead 340a of depolarizer 340. The third input lead of optical coupler 310 is notshown (as its end is tied off and/or crushed to minimize light that isreflected back into optical coupler 310). Depolarizer 340 significantlyreduces polarization-induced signal fading allowing inexpensive singlemode fiber to be used for all of the optical components and cable fibersrather than costly polarization-maintaining fiber. Depolarizer 340 maybe one of several commercially available depolarizers, such as, forexample, a recirculating coupler (single or multiple stage) or a LyotDepolarizer. Output lead 340 b of depolarizer 340 is connected to inputlead 330 b of optical coupler 330. First output lead 330 c of opticalcoupler 330 enters into transducer 200 (e.g., which may be any of thetransducers illustrated or described within the present application)through optical fiber 150. Optical fiber 150 is wrapped (e.g., a desirednumber of turns) between the fixed mandrel and the moveable mandrel(described above), and the distal end of optical fiber 150 terminates atreflector 350 (e.g., broadband reflector 350). As will be appreciated bythose skilled in the art, physical disturbances of the body of interestcause small changes in the length of fiber 150. These changes causenon-reciprocal changes in the phase of the light travelling through theSagnac interferometer, and the interferometer converts the phase changeof the light into an intensity change by allowing coherent interferencebetween the light traveling in two counterpropagating directions,recombining at the optical coupler 330. This intensity change in thelight is transmitted to optical receiver 304, where such intensitychange is interpreted as motion/acceleration/disturbance of the body ofinterest by processor 306 connected to optical receiver 304.

FIGS. 3B-3C illustrate Michelsen interferometer fiber optic sensingsystems 352, 380. System 352 illustrated in FIG. 3B includes internalmodulation at sensor 352 a, while system 380 illustrated at FIG. 3Bincludes external modulation (i.e., external of sensor 380 a).

Referring specifically to FIG. 3B, optical source 354 (e.g., a laser)transmits an optical signal (e.g., laser light) to optical circulator356. As will be appreciated by those skilled in the art, opticalcirculator 356 allows optical signals to pass only from port 1 to port2, and from port 2 to port 3. The optical signal generated from laser354 follows from port 1 to port 2, and along fiber length 360 withinlead cable 358. Upon exiting optical coupler 362 (e.g., a 1×2 opticalcoupler) the optical signal is split between transducer 200 (e.g., whichmay be any of the transducers illustrated or described within thepresent application) and phase modulator 376. As will be appreciated bythose skilled in the art, phase modulator 376 may include a referencecoil. The split optical signals pass through the fiber within transducer200 (including wound fiber length 150) and pass through phase modulator376 and then reflect at reflectors 364, 366. Reflectors may be, forexample, Faraday rotator mirrors. The reflected optical signalsrecombine (coherently) at optical coupler 362 and transmit back alongfiber 360 within lead cable 358 to port 2 of optical circulator 356.From port 2 the recombined signal follows to port 3 of opticalcirculator 356, and then to optical receiver 368 (e.g., a photodetectoror other optical detector). This recombined signal (which has a changein optical intensity which can be correlated to a disturbance of a bodyof interest) is converted at optical receiver 368 to electron hole pairsreceived by phase demodulator 370. Phase demodulator communicates withprocessor 372 for determination of the desired information related tothe physical disturbance of the body of interest. As will be appreciatedby those skilled in the art, phase demodulator 370 may generate a phasemodulation drive signal (e.g., a carrier voltage drive signal) alongwire 374 to phase modulator 376. That is, the power to control phasemodulator 376 is carried along wire 374 (e.g., twisted copper wires374).

FIG. 3C illustrates optical source 382 (e.g., a laser) that transmits anoptical signal (e.g., laser light) to external phase modulator 384. Theoptical signal exiting phase modulator enters port 1 of opticalcirculator 386 and exits through port 2. From port 2 the optical signalenters optical coupler 388 of sensor 380 a where the optical signal issplit. That is, the optical signal is split between transducer 200(e.g., which may be any of the transducers illustrated or describedwithin the present application) and reference coil 378. The splitoptical signals pass through the fiber within transducer 200 (includingwound fiber length 150) and through reference coil 378 and then theoptical signals reflect at reflectors 390, 398. Reflectors 390, 398 maybe, for example, Faraday rotator mirrors. The reflected optical signalsrecombine coherently at optical coupler 388 and transmit back alongfiber optic cable to port 2 of optical circulator 386. From port 2 therecombined signal follows to port 3 of optical circulator 386, and thento optical receiver 396 (e.g., a photodetector or other opticaldetector). This recombined signal (which has a change in opticalintensity which can be correlated to a disturbance of a body ofinterest) is converted at optical receiver 396 to electron hole pairsreceived by phase demodulator 392. Phase demodulator communicates withprocessor 394 for determination of the desired information related tothe disturbance of the body of interest. As will be appreciated by thoseskilled in the art, phase demodulator may generate a phase modulationdrive signal (e.g., a carrier voltage drive signal) along wire 392 a(e.g., a twisted copper wire) to phase modulator 384.

As provided above, each of the exemplary fiber optic sensing systems ofFIGS. 3A-3C includes a single sensor. Of course, it is often desirableto have fiber optic sensing systems with multiple sensors, for example,for sensing disturbances within a large area (and/or along a relativelylong length). FIGS. 4A-4B illustrate fiber optic sensing systemsincluding a plurality of sensors.

Referring specifically to FIG. 4A, a plurality of linearized Sagnacinterferometers (e.g., interferometer 300 from FIG. 3A) are included infiber optic sensing system 400. System 400 includes optical source 20,and optical receiver 60. Optical source 20 generates an optical signalin a pulsed mode with optical couplers 402 (e.g., 1×2 tap couplers 402)upstream of each interferometer 300, through optical circulator 404, toallow time division multiplexed operation, wherein return pulses fromeach interferometer 300 are received at optical receiver 60 at adifferent time. That is, tap couplers 402 are used to split the opticalsignal (e.g., the source light intensity) among severalinterferometers/sensors. The optical signal is pulsed, and returnsignals from each interferometer 300 return to optical receiver 60 atdifferent times, but in their respective order of location. The returnsignals include intensity information proportional to the disturbancemeasured by each interferometer 300, where the information is processedby processor 406.

FIG. 4B illustrates fiber optic sensing system 408 including TDM (timedivision multiplexing) interrogator 410 for generating an opticalsignal. In this exemplary configuration, FBGs (i.e., fiber bragggratings) are provided on each side of each transducer 200. Eachtransducer, and its surrounding FBGS, may be considered aninterferometer 414 a, 414 b, etc. Each of FBGs 412 a, 412 b, 412 c, etc.act as partial reflectors. Interrogator 410 (which typically includes anoptical source, a phase modulator, a light pulser, an optical receiver,and a phase demodulator) initiates an optical signal pulse that ispartially reflected by each FBG 412 a, 412 b, 412 c, etc. Lightreflected at pairs of each FBG 412 a, 412 b, 412 c, etc. is combinedcoherently. The combined signals arrive in time order such that TDMinterrogator 410 (in connection with processor 416) can determine thelight intensity change that results from the disturbance at eachtransducer 200.

FIG. 5 is an exploded view of transducer 200 n (previously described inconnection with FIGS. 2I-2J). FIG. 5 illustrates fiber 150 n entering(and exiting) transducer 200 n at the same point adjacent to top plate204 n. Top plate 204 n is secured to enveloping mass 220 n. Fiber 150 nis wound between moveable mandrel 210 n and fixed mandrel 205 n. Topplate 204 n is secured to moveable mandrel 210 n, and as such,enveloping mass 220 n is also secured to moveable mandrel 210 n throughtop plate 204 n. Biasing spring 230 n is disposed between fixed mandrel205 n and moveable mandrel 210 n. Bottom plate 208 n secures circularhinge 210 n to mass 220 n. Mounting plate 206 n secures the inner regionof circular hinge 210 n to fixed mandrel 205 n. As provided above,mounting plate 206 n may also be used to secure transducer 200 n to abody of interest (or to an interposing structure, not shown).

Circular hinge 210 n limits movement between objects attached to itsinner diameter (in FIG. 5, fixed mandrel 205 n is configured to beattached to the inner diameter of hinge 210 n) and its outer diameter(in FIG. 5, mass 220 n is configured to be attached to the outerdiameter of hinge 210 n). More specifically, circular hinge 210 nsubstantially limits relative motion between mass 220 n (secured tocircular hinge 210 n) and fixed mandrel 205 n to substantially linearmotion. Such linear motion may be along the “Y” axis described abovewith respect to FIGS. 2A-2H. FIGS. 6A-6B illustrate inner diameter 210 n1 and outer diameter 210 n 2 of circular hinge 210 n. FIG. 6Cillustrates inner diameter 210 n 1 secured to Structure #1, and outerdiameter 210 n 2 secured to Structure #2.

In certain transducers according to the present invention, it may bedesirable to provide for “damping” such as elastomeric damping, fluiddamping, etc. That is, it is often desirable to reduce the qualityfactor of a transducer resonance peak by absorbing energy in the form ofheat. Damping may also be used to increase the sensitivity of atransducer below its resonant frequency. FIG. 7A illustrates transducer200 m (similar in most respects to transducer 200 a in FIG. 2B, exceptthat the reference letter “a” has been replaced with reference letter“m”). A difference in transducer 200 m is the inclusion of elastomericshear damping elements 224 and elastomeric compression damping elements226. As shown in FIG. 7A, damping elements 224 are disposed betweenfixed mandrel 205 m and mass 220 m. Further, damping elements 226 areprovided between mass 220 m and body of interest 202 m. FIG. 7Bconceptually illustrates shear damping between mass 220 m and fixedmandrel 205 m, while FIG. 7C conceptually illustrates compressiondamping between mass 220 m and body of interest 202 m. Such dampingtechniques may be applied to each of the transducers illustrated anddescribed in the present application.

Exemplary applications for the transducers, accelerometers, and fiberoptic sensing systems of the present invention include vertical seismicprofiling (VSP), three dimensional sub-surface mapping, microseismicmonitoring, machine vibration monitoring, civil structure (e.g., dams,bridges, levees, buildings, etc.) monitoring, tunnel detection,perimeter/border security, earthquake monitoring, borehole leakdetection, roadbed erosion, railbed erosion, amongst others.

Although various exemplary transducers of the present invention aredescribed in connection with a fixed mandrel rigidly attached to a bodyof interest (or rigidly attached to the body of interest through a baseplate or other structure) it is not limited thereto. For example, ratherthan such a rigid attachment, the fixed portion may be magnetized (orinclude a magnetized portion) such that the fixed portion may be securedto the body of interest, where the body of interest includes a ferrousmaterial.

Although illustrated and described above with reference to certainspecific embodiments, the present invention is nevertheless not intendedto be limited to the details shown. Rather, various modifications may bemade in the details within the scope and range of equivalents of theclaims and without departing from the spirit of the invention.

What is claimed:
 1. A transducer comprising: a fixed portion configuredto be secured to a body of interest; a moveable portion having a rangeof motion with respect to the fixed portion; a spring positioned betweenthe fixed portion and the moveable portion; and a length of fiber woundbetween the fixed portion and the moveable portion, the length of fiberspanning the spring, wherein the fixed portion, the moveable portion,and the spring are formed from a unitary piece of material; a massengaged with the moveable portion, wherein the mass envelopes at leastone of the fixed portion, the spring, and the length of fiber within atleast one position within the range of motion of the moveable portion.2. The transducer of claim 1 wherein the mass is formed from the unitarypiece of material.
 3. The transducer of claim 1 wherein the unitarypiece of material is a plastic material.
 4. The transducer of claim 1wherein the unitary piece of material is a metal material.
 5. Thetransducer of claim 1 wherein the unitary piece of material is a springmetal material.
 6. The transducer of claim 1 wherein the spring is abiasing spring for providing tension to the length of fiber.
 7. Thetransducer of claim 1 further comprises a hinge connected between themass and the fixed portion to limit the range of motion of the moveableportion to a single linear degree of freedom.
 8. The transducer of claim7 wherein the hinge is formed from the unitary piece of material.
 9. Anaccelerometer comprising: a fixed portion configured to be secured to abody of interest; a moveable portion having a range of motion withrespect to the fixed portion; a spring positioned between the fixedportion and the moveable portion; and a length of fiber wound betweenthe fixed portion and the moveable portion, the length of fiber spanningthe spring, wherein the fixed portion, the moveable portion, and thespring are formed from a unitary piece of material; a mass engaged withthe moveable portion, wherein the mass envelopes at least one of thefixed portion, the spring, and the length of fiber within at least oneposition within the range of motion of the moveable portion.
 10. Theaccelerometer of claim 9 wherein the mass is formed from the unitarypiece of material.
 11. The accelerometer of claim 9 wherein the masshouses at least one optical element of the accelerometer.
 12. Theaccelerometer of claim 9 wherein the unitary piece of material is aplastic material.
 13. The accelerometer of claim 9 wherein the unitarypiece of material is a metal material.
 14. The accelerometer of claim 9wherein the unitary piece of material is a spring metal material. 15.The accelerometer of claim 9 wherein the spring is a biasing spring forproviding tension to the length of fiber.
 16. The accelerometer of claim9 further comprising a hinge connected between the mass and the fixedportion to limit the range of motion of the moveable portion to a singlelinear degree of freedom.
 17. The accelerometer of claim 16 wherein thehinge is formed from the unitary piece of material.
 18. A fiber opticsensing system comprising: an optical source for generating an opticalsignal; an optical element for dividing the optical signal generated bythe optical source, and for recombining optical signals received by theoptical element; an optical receiver for receiving a combined opticalsignal downstream of the optical element; a transducer including (1) afixed portion configured to be secured to a body of interest, (2) amoveable portion having a range of motion with respect to the fixedportion, and (3) a spring positioned between the fixed portion and themoveable portion, wherein the fixed portion, the moveable portion, andthe spring are formed from a unitary piece of material; and fiber opticcable extending between elements of the fiber optic sensing system, thefiber optic cable including a length of fiber wound between the fixedportion and the moveable portion, the length of fiber spanning thespring; a mass engaged with the moveable portion, wherein the massenvelopes at least one of the fixed portion, the spring, and the lengthof fiber within at least one position within the range of motion of themoveable portion.
 19. The fiber optic sensing system of claim 18 whereinthe fiber optic sensing system includes at least one of a Sagnacinterferometer sensing system, a Michelsen interferometer sensingsystem, and a Fabry Perot interferometer sensing system.